Coatings having specific release properties toward adhesives are widely used. Polydimethylsiloxanes and polymers containing predominately dimethylsiloxane units provide very low release coatings, e.g., they exhibit release of 4 to 16 grams per centimeter of width, for products such as labels or large sheets which contain normally tacky and pressure-sensitive adhesives. These polymers are less useful as release coatings on the back surface of adhesive tape (back surface coating of adhesive tape is known as "low adhesion backsize (LAB)) because their low release force can cause roll instability. LABs for tapes in roll form ideally exhibit release toward the adhesive of about 60 to 350 g/cm of width. Polymers with higher release values make it increasingly difficult to use the tape and delamination of the adhesive from the substrate often can result. Coatings having release values less than 60 g/cm of width are useful as components for release liners. Many non-silicone polymers, e.g., urethanes, find use as low adhesion backsizes for pressure-sensitive tapes because of their much higher release force than the polydimethylsiloxanes, typically greater than 200 g/cm width. Such non-silicone LAB coatings are exemplified in U.S. Pat. Nos. 2,607,711, 2,876,894, and 3,342,625.
For products such as tapes and liners, coatings having specific release properties toward adhesives, which are intermediate between those of the polydimethylsiloxanes and conventionally used non-silicone LAB coatings, are highly desired. Many previous attempts to provide such coatings by modification of polydimethyl siloxanes or blending them with less effective release material, as disclosed in, for example, U.S. Pat. Nos. 3,328,482, 3,527,659, 3,770,687, and 3,997,702 have not met with total success because of many problems as: (1) incompatibility of components causing extensive migration of the silicone to the surface, (2) contamination of the adhesive with low molecular weight components giving adhesion loss, (3) nonreproducibility, e.g., inability to consistently achieve the desired release level, and (4) use of excessively high cure temperatures causing deterioriation of the heat-sensitive substrate or tape backing.
U.S. Pat. No. 4,822,687 provides release compositions using epoxypolysiloxanes useful in the present invention.
Coating compositions that provide intermediate release (also called controllable release and differential release) properties towards adhesives, more recently, have been prepared from modifications of epoxypolysiloxanes either by introduction of organic groups into the backbone of the epoxypolysiloxane or by copolymerization of the epoxypolysiloxane with copolymerizable monomer. Epoxypolysiloxanes modified by introduction of polyoxyalkylene groups is described in U.S. Pat. No. 4,684,709; by introduction of phenolic compounds is described in U.S. Pat. Nos. 4,952,657 and 4,954,364; and by introduction of haloarylalkylsiloxy groups in U.S. Pat. No. 4,994,299.
Coating compositions providing differential release utilizing copolymerization of the epoxypolysiloxane with epoxysilanes is described in U.S. Pat. Nos. 4,279,717 and 4,313,988 and with a styrene compound in U.S. Pat. No. 4,988,741.
In U.S. Pat. No. 4,547,431 there are described differential release compositions utilizing copolymerization of epoxy functional polydiorganosiloxane silicone fluids (apparently limited to pre-crosslinked silicone fluids having up to about 20% by weight of epoxyfunctional groups) and a small amount (up to 20% by weight) of a polyfunctional epoxy monomer or mixture of monomers and photocatalyst that must be capable of dissolving or dispersing well in the epoxyfunctional silicone release composition. In U.S. Pat. No. 4,576,999 differential release compositions are described that utilize the copolymerization of precrosslinked epoxy-functional silicone fluids and epoxidized .alpha.-olefins having 11 through 14 carbon atoms and also to the copolymerization of these silicone fluids with diepoxides that must be miscible with both the epoxy-functional silicone fluid and the onium salt photocatalyst.